The present invention relates to a process for the purification of impure terephthalic acid produced by oxidation with air of p-xylene and/or the partially oxidized derivatives thereof, such as p-tolualdehyde and p-toluic acid. More particularly, it relates to the removal of such impurities as p-toluic acid and 4-carboxybenzaldehyde by crystallization at sufficiently high temperatures, without resorting to any chemical purification procedure such as post-oxidation, hydrogenation or decarbonylation.
It is a well known fact that terephthalic acid used as a starting material for the manufacture of polyester fibers and films must achieve a high degree of purity, since the presence of contaminants, even in minute amounts, during the polycondensation of terephthalic acid with ethylene glycol has pernicious consequences upon the quality of the resulting resin. For instance, monocarboxylic acid oxidation intermediates, such as p-toluic acid and/or 4-carboxybenzaldehyde, may react with ethylene glycol when present in the polycondensation reaction mixture and therefore act as a chain stopper, with the consequence that the melting point and strength of the resulting polyester may be substantially and undesirably lowered.
As a matter of fact, most processes disclosed in the prior art for the purification of terephthalic acid to be used for the manufacture of polyester fibers are directed to the removal therefrom of p-toluic acid and/or 4-carboxybenzaldehyde. To summarize the enormous amount of published material devoted to this commercially important problem, it can be stated that p-toluic acid is generally removed through controlled recrystallization of terephthalic acid under well defined conditions, whereas 4-carboxybenzadlehyde must be chemically transformed into another compound which is more easily separable from terephthalic acid by physical means. Thus, for example, 4-carboxybenzaldehyde is converted to p-toluic acid and/or p-methylolbenzoic acid through hydrogenation or benzoic acid through decarbonylation. For instance, in British Pat. No. 1,152,575 there is described a process which comprises treating an aqueous solution of impure terephthalic acid at high temperature (450.degree.-600.degree. F. or 232.degree.-316.degree. C.) in the presence of hydrogen with a platinum-charcoal hydrogenation catalyst to effect reduction of 4-carboxybenzaldehyde and recovering fiber-grade terephthalic acid by crystallization from the solution by controlled rate evaporation of water in a cascade of several crystallizers operated at decreasing temperatures. Terephthalic acid containing as low as 25 ppm of residual 4 -carboxybenzaldehyde can be obtained by this method.
In order to be purified efficiently to the fiber-grade, the crude terephthalic acid used as starting material in the procedure described hereabove must have a purity of at least 99% by weight, preferably 99.5%, with 4-carboxybenzaldehyde as the principal impurity. In fact, this procedure was especially designed for the purification of crude terephthalic acid obtained by liquid-phase oxidation of p-xylene at high temperature and pressure in the presence of acetic acid as a solvent and a bromine-containing compound as a promoter. Under such vigorous conditions, high reaction rates and high conversions are achieved with the advantageous results that terephthalic acid precursors such as p-toluic acid and 4-carboxylbenzaldehyde are present at relatively low concentrations in the reaction medium where terephthalic acid crystals are growing and therefore show little tendency to become occluded therein. However, this advantage is to some extent counterbalanced by the fact that the severity of the conditions used also results in serious corrosion problems and in partial burning of the acetic acid solvent. Consequently, combining a high-severity oxidation process with a relatively cumbersome purification procedure to remove as little as 0.5% of impurities increases more and more the production costs of fiber-grade terephthalic acid, inasmuch as energy and chemicals are becoming more expensive.
For this reason, several modifications of that oxidation process have recently been proposed, according to which the severity of the operating conditions is increased still more to obtain directly, i.e., without resorting to complicated and costly purification procedures, a terephthalic acid product which contains 300 to 1000 ppm of 4-carboxybenzaldehyde (medium-purity terephthalic acid) but which is still claimed to be suitable for the manufacture of polyester fibers. However, the increased severity of these modified processes results in increased corrosion problems and in increasing combustion rates of acetic acid, with the consequences that that which is gained on one side is at least partly lost on the other side.
Another approach to solve the problem is to reduce the production costs of crude terephthalic acid. As shown recently in U.S. patent application Ser. No. 030,054, it is possible to oxidize p-xylene into terephthalic acid with good yield and practical rates while using mild conditions and substituting water for acetic acid as a solvent. Combining this oxidation procedure with a purification method of the type mentioned above should result, at first glance, in substantial cuts in the production costs of fiber-grade terephthalic acid, especially because water could be used as the same solvent for both stages. However, in this water-diluted oxidation system, the concentration of p-toluic acid must inherently be maintained sufficiently high to allow the reaction to take place without a brominated promoter, with the result that substantial amounts of intermediate products become occluded into the crystals of terephthalic acid growing in the medium. Actually, these crystals may contain as much as 3-5% by weight of p-toluic acid and 2-2.5% of 4-carboxybenzaldehyde, and therefore they do not satisfy the conditions required for purification by the hydrogenation procedure described in the British patent referred to hereinabove.
To solve this other problem, it has been proposed in U.S. patent application Ser. No. 143,141 to modify the hydrogenation procedure by adding small amounts of various transition metals into the hydrogenation medium. In this way, the 4-carboxybenzaldehyde level can surprisingly be reduced from, e.g., 2%, down to less than 20 ppm in one step. However, since the resulting product may contain more than 5% of p-toluic acid, it is not possible to achieve the purification thereof by any crystallization procedure disclosed in the prior art. In order to do this, it would be necessary to first submit the hydrogenated product to some preliminary leaching, digestion or extraction with water at high temperature, which would result in additional processing costs. So, here again, in the converse manner, that which is gained on one side is partly lost on the other.